Fly ash is the by-product of burning pulverized coal. Wood ash is defined as the inorganic residue of wood combustion. Although the invention will be described primarily in connection with fly ash processing methods and apparatus, it can also be beneficially employed to reduce the carbon content of wood ash or for any particulate mixture in which it is desired to separate and remove carbon and light weight coarse fractions from the mixture.
The chemical content and particle size of fly ashes vary widely in accordance with the source of the coal, the fineness to which it is ground, and the furnace within which it is burned.
The chemical composition of fly ash may vary generally, as follows:
______________________________________ Class F Class C Component Percent by Weight Percent by Weight ______________________________________ SiO.sub.2 35-55 20-40 Al.sub.2 O.sub.3 15-35 5-15 FeO 3-25 1-10 Fe.sub.2 O.sub.3 CaO 0.5-8 0-35 MgO 0.5-3 0-5 TiO.sub.2 1-3 1-3 Na.sub.2 O 0-1.5 0-1.5 K.sub.2 O 0-3 0-3 SO.sub.3 1-3 0-6 C 0.5-20 0.5-20 H.sub.2 O 0.5-0.7 0.5-3 ______________________________________
Fly ash can be used for a variety of purposes. Principally, it is used as a replacement for cement in Portland Cement concrete. Additionally, for example, it has been used as filler material for plastics and asphalt, as a source of activated carbon for waste water purification, and as a magnetite source for coal cleaning.
Fly ash can also be used as a structural fill, soil additive and base for lime/fly ash, soil cement, or cementitious road way materials and the like. It is little wonder then that fly ash can provide a marketable commodity for utility companies and industrial plants that burn coal in order to generate electrical energy.
Many class F fly ashes collected throughout the United States have carbon content (as measured by Loss on Ignition--LOI) of about 0.5-4%. These ashes may be sold as acceptable mineral admixture for use in Portland cement concrete under the standards set forth in ASTM C-618-92a which limits maximum LOI for classes F and C fly ash at 6%. As such, these ashes are a commercially attractive commodity with the sale of same by electrical utilities and industrial concerns providing lucrative revenue.
Unfortunately, not all fly ashes possess such low carbon content as to pass the standards provided in ASTM C-618. These non-conforming fly ashes have LOI content of greater than 6%, sometimes on the order of &gt;6%-20%.
In addition to lack of conformity with the requirements of ASTM C-618, high carbon content fly ash has been shown to cause undesirable reduction in entrained air in concrete and it provides an oily or dark surface appearance on finished concrete surfaces since it floats to the surface during finishing. Moreover, high carbon content fly ash exhibits reduction in desirable pozzolanic reactivity.
Suffice it to say that use of fly ash that is not in conformity with the aforementioned ASTM standard as a mineral admixture in Portland Cement concrete and its consequent failure to meet material specifications raise serious liability problems.
Due to the limitations upon its ultimate end use, such high carbon fly ash also poses a disposal problem.
In light of the above, there exists a need in the industry to provide an inexpensive method and apparatus by which the carbon content (as measured by LOI) of high carbon containing fly ash mixtures can be reduced so that the resulting fly ash will conform with the above ASTM standard; therefore providing that the fly ash can be acceptably sold for and used as a mineral admixture for Portland Cement concrete products and other purposes.